1. Field of the Invention
The present invention relates generally to a light emitting diode (LED) based lighting apparatus, and more particularly to an LED driving circuit with low flicker and high power factor.
2. Description of Related Arts
LEDs are semiconductor-based light sources often employed in low-power instrumentation and appliance applications for indication purposes in the past. The application of LEDs in various lighting units has also become more and more popular. For example, high brightness LEDs have been widely used for traffic lights, vehicle indicating lights, and braking lights. In recent years, high voltage LED-based lighting apparatus have been developed to replace the conventional incandescent and fluorescent lamps.
In order to increase the brightness of an LED light, a number of LEDs are usually connected in series to form an LED-based lighting string and a number of LED-based lighting strings may further be connected in series to form a lighting apparatus. The operating voltage required by each lighting string typically is related to the forward voltage of the LEDs in each lighting string, how many LEDs are employed for each of the lighting string and how they are interconnected, and how the respective lighting strings are organized to receive power from a power source.
FIG. 1 shows a conventional LED-based lighting unit with a linear driving circuit 102. The LED-based light unit comprises a plurality of LED segments 110, 120 connected in series and controlled by the linear driving circuit 102. For simplicity, FIG. 1 only shows two segments 110 and 120. Each LED segment comprises one or more LEDs 103 connected in series. A rectified AC voltage source 101 provides power to the LED-based lighting unit.
FIG. 2 shows the voltage levels of the input AC voltage and the brightness of the LED-based lighting unit. The linear driving circuit 102 controls the number of segments that are turned on according to the rectified AC voltage. As a result, the brightness of the LED-based lighting unit is proportional to the rectified voltage level of the rectified AC voltage source. As can be seen in FIG. 2, the brightness of the LED-based lighting unit varies according to the variation of the input AC voltage and therefore has high flicker because the brightness changes significantly from zero to its maximum level. Because the rectified AC voltage output is not regulated, the linear driving circuit 102 is simple and requires low cost.
In order to reduce the brightness variation, a storage capacitor 301 as shown in FIG. 3 may be added to the LED-based lighting unit to regulate the voltage level of the rectified AC voltage output to form DC voltage. FIG. 3 also shows the voltage levels of the input AC voltage and the DC voltage after regulation as well as the brightness of the LED-based lighting unit. As can be seen, the lowest brightness of the LED-based lighting unit is increased significantly and the brightness variation is also greatly reduced.
In the conventional LED-based lighting unit shown in FIG. 3, the maximum AC current does not occur at the time when the input AC voltage reaches the maximum voltage level. FIG. 4 shows the values of the input AC voltage and the AC current. It can be seen that the AC current increases abruptly to start the charging phase and then linearly decreases to the discharging phase of the storage capacitor.
During the charging phase, the AC current drives the LEDs and also charges the storage capacitor. During the discharging phase, the LED current is supplied by the storage capacitor. From the waveform of the AC current, it can be seen that the waveform has high harmonic distortion due to the abrupt increase and then linear decrease in the AC current. As a result, the LED-based lighting unit has a low power factor (PF).